This study compares synchronous versus asynchronous online teaching in an introductory calculus-based physics class. Like many schools, our institution transitioned to online teaching during the COVID-19 pandemic. We held optional synchronous online sessions during the original class time, and approximately half of our students were regular attendees. In the study, we look at how students' grades changed after the transition. While students who attended the synchronous sessions saw an average test grade drop of 3.5%, students who did not attend saw a drop of 14.5%. The students who attended also saw a larger gain the standard Force Concept Inventory pre/post assessment: 56% versus 43%. We also created a survey to gauge the students' perceptions of the course difficulty and workload. The survey showed that students who did not attend the synchronous sessions found the course more difficult and felt they spent more time on the class than those who attended. These findings lead to our recommendation that future online offerings should require mandatory synchronous participation.